Copy src json file to build outputs

[chandlerprall]

Summary

Fixes #6171

Updated the build process to copy src/**/*.json files into each of the builds. Tested this in a fresh create react app; used v65.0.1 to verify failure and then pointed the app at a local eui build containing this fix.

Verified the files were copied as expected:

% find . -name logicals.json
./optimize/lib/global_styling/functions/logicals.json
./optimize/es/global_styling/functions/logicals.json
./test-env/global_styling/functions/logicals.json
./lib/global_styling/functions/logicals.json
./es/global_styling/functions/logicals.json
./src/global_styling/functions/logicals.json

Checklist

- [ ] Checked in both light and dark modes
- [ ] Checked in mobile
- [ ] Checked in Chrome, Safari, Edge, and Firefox
- [ ] Props have proper autodocs and playground toggles
- [ ] Added documentation
- [ ] Checked Code Sandbox works for any docs examples
- [ ] Added or updated jest and cypress tests

• Checked for breaking changes and labeled appropriately
  - [ ] Checked for accessibility including keyboard-only and screenreader modes
  - [ ] Updated the Figma library counterpart
• A changelog entry exists and is marked appropriately

[thompsongl]

Confirmed with a local build that the JSON files are in the right asset location

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6172/

[cee-chen]

Thanks for catching this!